** FILE **This undated family file photo provided by the Law Offices of John Burris shows Oscar Grant, a 22-year-old transit rider who was shot and killed by BART police on New Year's Day, 2009. A Douglas County jail official confirmed that 27-year-old Johannes Mehserle, who was involved in the incident, was in custody Tuesday night, Jan. 14, 2009, under a fugitive warrant issued in California (AP Photo/Family Handout provided by the Law Offices of John Burris) ** NO SALES **

BART officials said Saturday that they will investigate the actions of one of the transit agency's police officers after a video surfaced showing the officer striking a passenger - apparently Oscar Grant - minutes before the unarmed young man was fatally shot by another officer early on New Year's Day.

The cell phone video, one of a handful that have surfaced, aired Friday night on KTVU-TV. It shows a male BART police officer walking over to three men lined up against a wall near a female officer, and then striking one in the face.

The victim of the punch - identified by Channel 2 as 22-year-old Grant - slides to the ground. The video then shows the moments preceding the shooting, then the shooting itself. It appears that the officer who punches the man is the same person who later is seen kneeling on Grant's head when he was shot.

Sources have identified that officer as Tony Pirone. He and the other officers present at the time of Grant's shooting all remain on paid administrative leave while the investigation continues, but until Saturday BART was not investigating the conduct of anyone besides Johannes Mehserle, 27, who shot Grant.

Mehserle later resigned from the force and was charged with murder. He pleaded not guilty and is being held without bail.

Police investigators have said Grant put up a brief struggle with officers but was restrained and had both arms behind his back when he was shot. It was not clear whether the struggle investigators referred to was the same incident caught on the new video.

But Peter Keane, a professor at Golden Gate University Law School and police expert, said Saturday night the video shows a "vicious, unprovoked and inexcusable assault" by the other police officer that should be prosecuted and that seems to have set off events that led to the shooting.

"With that powerful punch, he slams Mr. Grant in the side of his head and knocks him down even though it doesn't appear Grant is doing anything but talking - maybe he is mouthing off but there was no physical provocation," Keane said. "Why is the other officer not being prosecuted? It was a clearly a deliberate, abusive use of force."

He said the video is damning to Mehserle, too.

In a written statement released Saturday evening, BART General Manager Dorothy Dugger said she takes the "new allegation of police use of unreasonable force extremely seriously" and has directed BART Police Chief Gary Gee to conduct an internal affairs investigation. The officer was not identified in the statement.

Noting that the investigation into Grant's shooting is ongoing, Dugger vowed to "fully examine" any new evidence and urged KTVU to provide the agency and Alameda County district attorney's office - which is prosecuting Mehserle - with the video.

BART's board of directors voted unanimously on Jan. 12 to create a special committee that will provide additional oversight of the agency's Police Department.

In the same statement released Saturday, BART board Director Carole Ward Allen - who chairs the committee - insisted that the agency does not tolerate police misconduct and will hold its police force to the "highest standards of professionalism and ethical conduct."